After decades of excessive growth in health care spending, few would disagree that U.S. healthcare must be reorganized to achieve the same or better patient outcomes at lower cost. As the key decision makers in healthcare, physicians are in large part responsible for growth in healthcare utilization through ordering of imaging, diagnostic tests, procedures, and hospital services. And yet, much remains to be learned about variation in physician practice patterns and the implications of those differences for the outcomes of their patients. More broadly, the body of research quantifying the variation in clinical and economic outcomes across individual physicians is sparse and a greater understanding of the various individual physician and institutional factors that may influence that variation is needed. By combining economic methods with carefully chosen clinical scenarios, I will build a long-term research agenda that seeks to broadly understand factors determining physician practice patterns; the implications of individual practice variations for patient outcomes; and the impact of selected institutional and environmental factors on clinical and economic outcomes of physicians. By linking together a unique set of databases (medical claims of Medicare beneficiaries; demographic and medical training information of the physicians caring for these patients; and malpractice claims of these physicians), the proposed research will comprehensively study the variation and determinants of physician spending, quality of care, and patient outcomes, i.e. the 'anatomy' of physician behavior. In addition to quantifying variation in clinical and economic outcomes across physicians, the proposed research will, for a carefully chosen set of physician activities, assess: (1) how spending, qualit of care, and outcomes relate to physician age, sex, medical training, and practice environment, (2) whether clinical outcomes for physicians that spend more on average are better than those that spend less, (3) how a physician's own malpractice history affects his/her subsequent quality of care, spending, and outcomes, (4) how changes in medical training environments (e.g. mandated reductions in weekly work hours of resident physicians) impact physicians later in their career, and (5) how physician practice patterns respond to the local economic environments in which they practice. The ultimate goal of this research is to provide health policy researchers, physicians, and policymakers the scientific basis for tangible physician-based policies to improve quality of care and reduce wasteful health care spending.